In recent years, for the purpose of observing cerebral activity status, an optical cerebral function imaging apparatus capable of conveniently measuring cerebral activity status using lights in a noninvasive manner has been developed. In such an optical cerebral function imaging apparatus, a light-transmitting probe arranged on a scalp surface of a subject irradiates near-infrared light of three different types of wavelengths λ1, λ2, λ3 (e.g., 780 nm, 805 nm, 830 nm) on to a brain. While, a light-receiving probe arranged on the scalp surface detects an intensity change (received light quantity information) ΔA(λ1), ΔA(λ2), ΔA(λ3) of each of the near-infrared lights of wavelengths λ1, λ2, λ3 reflected from the brain.
In order to obtain the product of the change in oxyhemoglobin concentration and the optical path length [oxyHb] in a cerebral blood flow and the product of the change in deoxyhemoglobin and the optical path length [deoxyHb] from received light quantity information ΔA(λ1), ΔA(λ2), ΔA(λ3) obtained as mentioned above, equations (1), (2) and (3) using a Modified Beer Lambert law may be solved. Further, from the product of the change in oxyhemoglobin concentration and the optical path length [oxyHb] and the product of the change in deoxyhemoglobin and the optical path length [deoxyHb], a product of the change in total hemoglobin and the optical path length ([oxyHb]+[deoxyHb]) is calculated.ΔA(λ1)=EO(λ1)×[oxyHb]+Ed(λ1)×[deoxyHb]  (1)ΔA(λ2)=EO(λ2)×[oxyHb]+Ed(λ2)×[deoxyHb]  (2)ΔA(λ3)=EO(λ3)×[oxyHb]+Ed(λ3)×[deoxyHb]  (3)
E0(λm) denotes an absorbance coefficient of oxyhemoglobin in the light having a wavelength λm, and Ed(λm) denotes an absorbance coefficient of deoxyhemoglobin in the light having a wavelength λm.
Here, the relation between a distance (channel) between a light-transmitting probe and a light-receiving probe and a measurement position will be explained. FIGS. 7A and 7B show a relation between a pair of light-transmitting probe and light-receiving probe and a measurement position. A light-transmitting probe 12 is pressed into contact with a light-transmitting point T on a scalp surface of a subject, while a light-receiving probe 13 is pressed into contact with a light-receiving point R on the scalp surface of the subject. The light-transmitting probe 12 irradiates lights, and the light-receiving probe 13 receives lights reflected back to the scalp surface. At this time, lights passed through a banana-shaped region (measurement region) among lights irradiated from the light-transmitting point T on the scalp surface reach the light-receiving point R on the scalp surface. That is, the lights pass through blood vessels existing in the skin close to the light-transmitting point T, blood vessels existing in the brain, and blood vessels existing in the skin close to the light-receiving point R.
Under the circumstances, in order to obtain received light information ΔA relating only to blood vessels existing in the brain, a distance (channel) between the light-transmitting probe 12 and the light-receiving probe 13 is set to a short distance r1 and a distance (channel) between the light-transmitting probe 12 and the light-receiving probe 13 is set to a long distance r2 (see, e.g., Patent Document 1 or non-Patent Document 1). FIG. 8 is a cross-sectional view showing a relation between a reference light-receiving probe 14 arranged at a short distance r1 away from the light-transmitting probe 12, a light-receiving probe 13 arranged at a long distance r2 away from the light-transmitting probe 12 and measurement positions. With this, from the channel having the long distance r2, light quantity information ΔA2 is received. Light quantity information ΔA2 relates to blood vessels existing in the skin close to the light-transmitting point T, blood vessels existing in the brain, and blood vessels existing in the skin close to the light-receiving point R2. From the channel having the short distance r1, light quantity information ΔA1 relating only to blood vessels existing in the skin close to the light-transmitting point T (blood vessels existing in the skin close to the light-receiving point R1) is received.
From the received light quantity information ΔA1 and ΔA2 obtained as mentioned above, using the following Equation (4), received light quantity information ΔA relating only to blood vessels existing in the brain is obtained.ΔA=ΔA2−KΔA1  Equation (4)
In Equation (4), in order to obtain the received light quantity information ΔA, it is necessary to specify the coefficient(s) K. See, for example, Non-Patent Document 2 for a method for calculating the coefficient(s) K is disclosed. In this calculation method, the coefficient(s) K is calculated using a least square error.
Further, in an optical cerebral function imaging device, it is performed to respectively measure a product of the density change of oxyhemoglobin and the optical path length [oxyHb], a product of the density change of deoxyhemoglobin and the optical length [deoxyHb], and a product of the density change of total hemoglobin and the optical length ([oxyHb]+[deoxyHb]), which relate to a plurality of measuring points of the brain.
In such an optical cerebral function imaging device, in order to bring eight (8) light-transmitting probes 12 and eight (8) light-receiving probes 13 into contact with a scalp surface of a subject in a predetermined arrangement, a holder (light-transmitting/receiving unit) 130 is used. FIG. 9 is a plan view showing one example of the holder 130 into which eight (8) light-transmitting probes and eight (8) light-receiving probes are inserted.
The light-transmitting probes 12T1 to 12T8 and light-receiving probes 13R1 to 13R8 are arranged alternately such that four (4) probes are arranged in the lengthwise direction and four (4) probes are arranged in the lateral direction. In this arrangement, the second setting distance r2 which is a distance (channel) between the light-transmitting probe 12T1 to 12T8 and the light-receiving probe 13R1 to 13R8 is set to 30 mm. With this, received light quantity information ΔA2(λ1), ΔA2(λ2), and ΔA2(λ3) relating to twenty-four (24) measurement positions on a brain are obtained.